1. Field of the Invention
The present invention relates to an ultra-fast amplifier that is made with bipolar transistors and has good dynamic characteristics. These characteristics are obtained by means of a controlled positive feedback in current.
There are known ways, at present, of making operational amplifiers having very high gains of up to 100 million in low-frequency open-loop configurations, these gains being associated with very low offset voltages of some microvolts or some tens of microvolts.
These valuable characteristics are obtained at the cost of the dynamic performance characteristics: the gain-band product or transition frequency F.sub.T does not exceed some tens of megahertz and the slew-rate remains instrinsically limited to some tens of volts per microsecond except with highly sophisticated (and hence costly) technologies.
This deterioration of the dynamic performance characteristics results from the presence in the amplifier of a compensation capacitance C which determines the stability by creating a dominant pole that reduces the transition frequency and by limiting the slew-rate to I/C, I being the biasing current for the input stage.
The amplification of fast signals with restitution of the DC component needed in certain signal processing applications such as those related to video amplifiers, pulse amplifiers, fast sample-and-hold units etc. therefore cannot be resolved by this type of component.
In these applications, it is necessary, on the contrary, to have amplifiers with very wide passband (of some 100 MHz) and very high slew-rate (some 100 V/.mu.s) to amplify the fast signals without distortion. The precision required in DC mode is generally less stringent (1/100 or 1/1000) and a voltage offset of some millivolts is generally tolerated.
2. Description of the Prior Art
The prior art approaches to solving these difficulties fall into three categories. In the first category, the structures derived from the conventional techniques of operational amplifiers are based on dielectrical insulation technologies. These approaches do not allow for achieving the highest slew-rates.
The second category relates to structures based on the techniques of non-looped wideband amplifiers. The absence of any overall feedback eliminates the compensation capacitances. These amplifiers which are not subjected to feedback are intrinsically the fastest ones, those that make it possible to approach the limits of speed possible according to the technology. However, the restitution of the DC component is very difficult to obtain for it is based on a technique of compensation for the DC offsets provided by the successive stages This method, which is still an approximate one, is greatly subject to temperature drifts and does not give satisfactory results, notably because of the increase in the number of components that have be introduced in order to return to the DC level.
The third category comprises the amplifiers for which the high gain is obtained by a positive feedback in voltage. The output signal is available only under high impedance. This makes the circuit sensitive, in stability, to the capacitive loads. Furthermore, these circuits have a smaller natural phase margin. This necessitates a slight compensation, hence a smaller passband and a reduced slew-rate, and makes it difficult in certain cases to achieve looping in follower mode.